The evolution of the eukaryotic cell was accompanied by several endosymbioses (Embley and Martin 2006). Mitochondria, hydrogenosomes and mitosomes, providing the host cell with ion sulfur clusters (Lill 2009) and, with the exception of mitosomes, ATP, originated from an engulfed bacterium (Lithgow and Schneider 2010). Additionally, plastids capable of photosynthesis developed from a cyanobacterium-like cell, which was intracellularly reduced to an organelle (Hempel et al. 2007; Bolte et al. 2009). Thus, major cellular functions and organelles of eukaryotes are of bacterial origin. This makes the analysis of further bacterial endosymbioses in eukaryotic cells an attractive field for studying the conversion of a prokaryote into a eukaryotic organelle. Several eukaryotes are known to harbor intracellular prokaryotes which were acquired via endosymbiotic events that occurred later than those giving rise to mitochondria and plastids. In the case of more recently established intracellular cyanobacteria, most of the described taxa are not cultivated, making them difficult to analyze. However, one important exception is the spheroid body harboring diatom Rhopalodia gibba, which can be isolated and maintained for some time in culture (Adler et al. 2010).